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1.
The sorption of Np(V) and Np(IV) onto kaolinite has been studied in the absence and presence of humic acid (HA) in a series of batch equilibrium experiments under different experimental conditions: [Np]0: 1.0 × 10-6 or 1.0 × 10-5 M, [HA]0: 0 or 50 mg/L, I: 0.01 or 0.1 M NaClO4, solid to liquid ratio: 4 g/L, pH: 6–11, anaerobic or aerobic conditions, without or with carbonate. The results showed that the Np(V) sorption onto kaolinite is affected by solution pH, ionic strength, Np concentration, presence of carbonate and HA. In the absence of carbonate, the Np(V) uptake increased with pH up to ∼96% at pH 11. HA further increased the Np(V) sorption between pH 6 and 9 but decreased the Np(V) sorption between pH 9 and 11. In the presence of carbonate, the Np(V) sorption increased with pH and reached a maximum of 54% between pH 8.5 and 9. At higher pH values, the Np(V) sorption decreased due to the presence of dissolved neptunyl carbonate species with a higher negative charge that were not sorbed onto the kaolinite surface which is negatively charged in this pH range. HA again decreased the Np(V) uptake in the near-neutral to alkaline pH range due to formation of aqueous neptunyl humate complexes. The decrease of the initial Np(V) concentration from 1.0 × 10−5 M to 1.0 × 10−6 M led to a shift of the Np(V) adsorption edge to lower pH values. A higher ionic strength increased the Np(V) uptake onto kaolinite in the presence of carbonate but had no effect on Np(V) uptake in the absence of carbonate.  相似文献   

2.
Adsorption of Cr(VI) on γ-alumina was investigated as a function of ionic strength (0.001, 0.01 and 0.1 M NaNO3), pH (4-10), Cr(VI) concentration (10−4 or 10−5 M with 5 g/L solid) and pCO2 (0, atmospheric, 2.5%). Cr(VI) sorption is significant at low pH and decreases with increasing pH, with 50% of the Cr(VI) adsorbed between pH ∼6.5 and 8. Adsorption varies little with ionic strength or pCO2 under most of the studied conditions. However, at low pH under high ionic strength and especially at high ionic strength and high pCO2, Cr(VI) sorption on γ-alumina is suppressed. The adsorption edge data were used to parameterize constant capacitance (CCM), diffuse double layer (DLM) and triple layer (TLM) surface complexation models. None of the models entirely captures the full range of observed adsorption dependence on ionic strength and sorbate/sorbent ratio. The best fits to the full dataset are produced by the CCM, mostly because it has ionic-strength dependent stability constants. The more sophisticated TLM, which requires the most fitting parameters, does not produce better fits than the simpler CCM or DLM approaches for the conditions tested in this study.  相似文献   

3.
Neptunium is one of the few radioactive elements that are of great concern in the disposal of nuclear wastes in the geological repository, due to its hazards and the long half-life of the isotope, 237Np (t1/2 = 2.14 × 106 years). To understand and predict the migration behavior of neptunium in the geological media, it is of importance to study its hydrolysis at elevated temperatures, because the temperature in the waste package and the vicinity of the repository could be high. Moreover, the chemical analogy between neptunium(V) and plutonium(V) adds even greater value to this investigation, because the latter could exist at tracer levels in neutral and slightly oxidizing waters but is difficult to study due to its rather labile redox behavior.In this work, the hydrolysis of neptunium(V) was studied at variable temperatures (10 to 85°C) in tetramethylammonium chloride (1.12 mol kg−1). Two hydrolyzed species of neptunium(V), NpO2OH(aq) and NpO2(OH)2−, were identified by potentiometry and Near-IR absorption spectroscopy. The hydrolysis constants (*βn) and enthalpy of hydrolysis (ΔHn) for the reaction NpO2+ + nH2O = NpO2(OH)n(1−n)+ + nH+ (n = 1 and 2) were determined by titration potentiometry and microcalorimetry. The hydrolysis constants, *β1 and *β2, increased by 0.8 and 3.4 orders of magnitude, respectively, as the temperature was increased from 10 to 85°C. The enhancement of hydrolysis at elevated temperatures is mainly due to the significant increase of the degree of ionization of water as the temperature is increased. The hydrolysis reactions are endothermic but become less endothermic as the temperature is increased. The heat capacities of hydrolysis, ΔCp1 and ΔCp2, are calculated to be −(71 ± 17) J K−1 mol−1 and −(127 ± 17) J K−1 mol−1, respectively. Approximation approaches to predict the effect of temperature, including the constant enthalpy approach, the constant heat capacity approach and the DQUANT equation, have been tested with the data.  相似文献   

4.
《Applied Geochemistry》2000,15(7):953-973
The enhancement of mobility of radionuclides in the geosphere through complexation by humic substances is a source of uncertainty in performance assessment of radioactive waste repositories. Only very few data sets are available which are relevant for performance assessment of an underground repository for radioactive waste. Using the equilibrium dialysis-ligand exchange method developed at the Paul Scherrer Institut, conditional stability constants for the formation of complexes of Aldrich humic acid with Ca2+, NpO2+, Co2+, Ni2+, UO22+ and Eu3+ and complexes of Laurentian soil- and Suwannee River fulvic acid with Co2+, UO22+ and Eu3+ were measured. pH was varied between 5 and 10 and ionic strength between 0.02 and 0.2 M. The data are presented as equilibrium coefficients that are free from any model assumptions. The equilibrium coefficients increased in the order Ca2+≅NpO2+<Co2+< Ni2+<UO22+< Eu3+. The quality of the data is assessed in an extended discussion of statistical and systematical errors, and by a critical ‘rereview’ of the auxiliary stability constants used for the calculation of the equilibrium coefficients. An approximate overall uncertainty of 0.5 log-units is estimated for the stability data reported. The conditional stability constants were found to increase markedly with increasing pH in the case of Co2+, UO22+ and Eu3+. For Ni2+, Ca2+ and NpO2+ this effect was less pronounced. For all metal ions tested, the influence of ionic strength was of less importance, and the conditional stability constants did not show a significant dependence on the type of humic substances investigated.  相似文献   

5.
Potentiometric measurements of Yttrium and Rare Earth Element (YREE) complexation by carbonate and bicarbonate indicate that the quality of carbonate complexation constants previously obtained via solvent exchange analyses are superior to characterizations obtained using solubility and adsorptive exchange analyses. The results of our analyses at 25°C are combined with the results of previous solvent exchange analyses to obtain YREE carbonate complexation constants over a wide range of ionic strength (0 ≤ I ≤3 molal). YREE carbonate complexation constants are reported for the following equilibria, M3++nHCO3−?M(CO3)n3−2n+nH+, where n = 1 or 2. Formation constants written in terms of HCO3− concentrations require only minor corrections for ion pairing relative to the corrections required for constants expressed in terms of CO32− concentrations. Formation constants for the above complexation equilibria, CO3Hβ1=[MCO3+][H+][M3+]−1[HCO3−]−1 and CO3Hβ2=[M(CO3)2−][H+]2[M3+]−1[HCO3−]−2, have very similar dependencies on ionic strength because the reaction MCO3++HCO3−?M(CO3)2−+H+ is isocoulombic. Potentiometric analyses indicate that the dependence of logCO3Hβ1 and logCO3Hβ2 on ionic strength at 25°C is given as
(A)  相似文献   

6.
Dissolution and precipitation rates of brucite (Mg(OH)2) were measured at 25°C in a mixed-flow reactor as a function of pH (2.5 to 12), ionic strength (10−4 to 3 M), saturation index (−12 < log Ω < 0.4) and aqueous magnesium concentrations (10−6 to 5·10−4 M). Brucite surface charge and isoelectric point (pHIEP) were determined by surface titrations in a limited residence time reactor and electrophoretic measurements, respectively. The pH of zero charge and pHIEP were close to 11. A two-pK, one site surface speciation model which assumes a constant capacitance of the electric double layer (5 F/m2) and lack of dependence on ionic strength predicts the dominance of >MgOH2+ species at pH < 8 and their progressive replacement by >MgOH° and >MgO− as pH increases to 10-12. Rates are proportional to the square of >MgOH2+ surface concentration at pH from 2.5 to 12. In accord with surface speciation predictions, dissolution rates do not depend on ionic strength at pH 6.5 to 11. Brucite dissolution and precipitation rates at close to equilibrium conditions obeyed TST-derived rate laws. At constant saturation indices, brucite precipitation rates were proportional to the square of >MgOH2+ concentration. The following rate equation, consistent with transition state theory, describes brucite dissolution and precipitation kinetics over a wide range of solution composition and chemical affinity:
  相似文献   

7.
Proton binding constants for the edge and basal surface sites of kaolinite were determined by batch titration experiments at 25 °C in the presence of 0.1 M, 0.01 M and 0.001 M solutions of NaNO3 and in the pH range 3-9. By optimizing the results of the titration experiments, the ratio of the edge sites to the basal surface sites was found to be 6:1. The adsorption of Cd(II), Cu(II), Ni(II), Zn(II) and Pb(II) onto kaolinite suspensions was investigated using batch adsorption experiments and results suggested that in the lower pH range the metallic cations were bound through non-specific ion exchange reactions on the permanently charged basal surface sites (X−). Adsorption on these sites was greatly affected by ionic strength. With increasing pH, the variable charged edge sites (SOH) became the major adsorption sites and inner-sphere specifically adsorbed monodentate complexes were believed to be formed. The effect of ionic strength on the extent of adsorption of the metals on the variable charged edge sites was much less than those on the permanently charged sites. Two binding constants, log K(X2Me) and log K(SOMe), were calculated by optimizing these constants in the computer program FITEQL. A model combining non-specific ion exchange reactions and inner-sphere specific surface complexations was developed to predict the adsorption of heavy metals onto kaolinite in the studied pH range. Linear free energy relationships were found between the edge site binding constants and the first hydrolysis constants of the metals.  相似文献   

8.
The effect of temperature on the sorption of cations onto a dioctahedral smectite was investigated by running batch experiments at 25, 40, 80 and 150°C. We measured the distribution coefficient (Kd) of Cs+, Ni2+ and 14 lanthanides (Ln3+) between solutions and the montmorillonite fraction of the MX80 bentonite at various pH and ionic strengths. Up to 80°C we used a conventional experimental protocol derived from Coppin et al. (2002). At 150°C, the experiments were conducted in a PTFE reactor equipped with an internal filter allowing the sampling of clear aliquots of solution.The results show a weak but measurable influence of the temperature on the elements sorption. Kd’s for Ni2+ and Ln3+ increase by a factor 2 to 5 whereas temperature raises from 25 to 150°C. This effect seems higher at high ionic strength. The estimated apparent endothermic sorption enthalpies are 33 ± 10 kJ.mol−1 and 39 ± 15 kJ.mol−1 for Ni2+ and Eu3+, respectively. On the other hand, the temperature effect on Cs+ sorption is only evidenced at low ionic strength and under neutral conditions where the Kd decreases by a factor 3 between 25 and 150°C. Apparent exothermic sorption enthalpy for Cs+ on the montmorillonite is −19 ± 5 kJ.mol−1.Experiments conducted at the four temperatures with the coexistence of all of the cations in the reacting solution (100 ppb of each element in the starting solution) or only one of them, produced similar values of Kd. This suggests the absence of competition between the sorbed cations, and consequently a low degree of saturation of the available sites. A fractionation of the lanthanides spectrum is also observed at high pH and high ionic strength whatever the temperature.The conclusion of this study is that the temperature dependence on sorption reflects, as the fractionation of REE or the pH and ionic strength effects, the chemical process which controls the overall reaction. In the case of an exchange dominated reaction (low pH and low ionic strength), the temperature effect is negligible. In the case of surface complexation (high pH and high ionic strength), the observed increase of Kd with temperature reflects either an increase of the sorption equilibrium constant with temperature or an endothermic property for reactions describing the montmorillonite surface chemistry.  相似文献   

9.
Numerous studies have utilized surface complexation theory to model proton adsorption behaviour onto mesophilic bacteria. However, few experiments, to date, have investigated the effects of pH and ionic strength on proton interactions with thermophilic bacteria. In this study, we characterize proton adsorption by the thermophile Anoxybacillus flavithermus by performing acid-base titrations and electrophoretic mobility measurements in NaNO3 (0.001-0.1 M). Equilibrium thermodynamics (Donnan model) were applied to describe the specific chemical reactions that occur at the water-bacteria interface. Acid-base titrations were used to determine deprotonation constants and site concentrations for the important cell wall functional groups, while electrophoretic mobility data were used to further constrain the model. We observe that with increasing pH and ionic strength, the buffering capacity increases and the electrophoretic mobility decreases. We develop a single surface complexation model to describe proton interactions with the cells, both as a function of pH and ionic strength. Based on the model, the acid-base properties of the cell wall of A. flavithermus can best be characterized by invoking three distinct types of cell wall functional groups, with pKa values of 4.94, 6.85, and 7.85, and site concentrations of 5.33, 1.79, and 1.42 × 10−4 moles per gram of dry bacteria, respectively. A. flavithermus imparts less buffering capacity than pure mesophilic bacteria studied to date because the thermophile possesses a lower total site density (8.54 × 10−4 moles per dry gram bacteria).  相似文献   

10.
The fate and transport of uranium in contaminated soils and sediments may be affected by adsorption onto the surface of minerals such as montmorillonite. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate the adsorption of uranyl (UO22+) onto Wyoming montmorillonite. At low pH (∼4) and low ionic strength (10−3 M), uranyl has an EXAFS spectrum indistinguishable from the aqueous uranyl cation, indicating binding via cation exchange. At near-neutral pH (∼7) and high ionic strength (1 M), the equatorial oxygen shell of uranyl is split, indicating inner-sphere binding to edge sites. Linear-combination fitting of the spectra of samples reacted under conditions where both types of binding are possible reveals that cation exchange at low ionic strengths on SWy-2 may be more important than predicted by past surface complexation models of U(VI) adsorption on related montmorillonites. Analysis of the binding site on the edges of montmorillonite suggests that U(VI) sorbs preferentially to [Fe(O,OH)6] octahedral sites over [Al(O,OH)6] sites. When bound to edge sites, U(VI) occurs as uranyl-carbonato ternary surface complexes in systems equilibrated with atmospheric CO2. Polymeric surface complexes were not observed under any of the conditions studied. Current surface complexation models of uranyl sorption on clay minerals may need to be reevaluated to account for the possible increased importance of cation exchange reactions at low ionic strengths, the presence of reactive octahedral iron surface sites, and the formation of uranyl-carbonato ternary surface complexes. Considering the adsorption mechanisms observed in this study, future studies of U(VI) transport in the environment should consider how uranium retardation will be affected by changes in key solution parameters, such as pH, ionic strength, exchangeable cation composition, and the presence or absence of CO2.  相似文献   

11.
《Applied Geochemistry》1999,14(5):569-579
Potential human intrusion into the Waste Isolation Pilot Plant (WIPP) might release actinides into the Culebra Dolomite where sorption reactions will affect of radiotoxicity from the repository. Using a limited residence time reactor the authors have measured Ca, Mg, Nd adsorption/exchange as a function of ionic strength, PCO2, and pH at 25°C. By the same approach, but using as input radioactive tracers, adsorption/exchange of Am, Pu, U, and Np on dolomite were measured as a function of ionic strength, PCO2, and pH at 25°C. Metal adsorption is typically favored at high pH. Calcium and Mg adsorb in near-stoichiometric proportions except at high pH. Adsorption of Ca and Mg is diminished at high ionic strengths (e.g., 0.5M NaCl) pointing to association of Na+ with the dolomite surface, and the possibility that Ca and Mg sorb as hydrated, outer-sphere complexes. Sulfate amplifies sorption of Ca and Mg, and possibly Nd as well. Exchange of Nd for surface Ca is favored at high pH, and when Ca levels are low. Exchange for Ca appears to control attachment of actinides to dolomite as well, and high levels of Ca2+ in solution will decrease Kds. At the same time, to the extent that high PCO2s increase Ca2+ levels, Kds will decrease with CO2 levels as well, but only if sorbing actinide-carbonate complexes are not observed to form (Am-carbonate complexes appear to sorb; Pu-complexes might sorb as well. U-carbonate complexation leads to desorption). This indirect CO2 effect is observed primarily at, and above, neutral pH. High NaCl levels do not appear to affect to actinide Kds.  相似文献   

12.
Comparative observations of PdII and PtII hydrolysis in chloride solutions indicate that [PdCl3OH2−]/[PdCl42−] and [PtCl3OH2−]/[PtCl42−] concentration ratios in salinity 35 seawater (S = 35) are smaller than one at a typical surface ocean pH (∼8.2), and are larger than one at pH = 8.2 when S < 10. The hydrolysis behaviors of PdCl42− and PtCl42− are very similar. In 0.5 M NaCl at 25°C the hydrolysis constant for both elements, written in the form β1 = [MCl3OH2−][Cl−][H+][MCl42−]−1, is logβ1*=−8.97. Between ionic strengths 0.3 M and 1.0 M for PdII and between 0.1 M and 1.0 M for PtII, log β1 is within approximately 0.1 units of the value appropriate to 0.5 M NaCl. This small dependence of PdCl42− and PtCl42− hydrolysis constants on ionic strength is consistent with predictions based on expected activity coefficient behavior.Carbonate is observed to complex PdCl42− significantly, but to a smaller extent than OH− under conditions appropriate to seawater. Complexation of PtCl42− by CO32− was observed in this work but the rate of complexation was too slow to allow equilibrium observations. The principal dissimilarity between the chemistries of PdII and PtII in our investigation was the sharp contrast in observed PdII and PtII reaction rates. Differences in reaction kinetics may cause fractionation of PdII and PtII in the environment. The speciation of PtII, unlike PdII, is likely to be based on chemical environments experienced by PtII over a period of days, and perhaps weeks.  相似文献   

13.
Uranyl adsorption was measured from aqueous electrolyte solutions onto well-characterized goethite, amorphous ferric oxyhydroxide, and hematite sols at 25°C. Adsorption was studied at a total uranyl concentration of 10?5 M, (dissolved uranyl 10?5 to 10?8 M) as a function of solution pH, ionic strength and electrolyte concentrations, and of competing cations and carbonate complexing. Solution pHs ranged from 3 to 10 in 0.1 M NaNO3 solutions containing up to 0.01 M NaHCO3. All the iron oxide materials strongly adsorbed dissolved uranyl species at pHs above 5 to 6 with adsorption greatest onto amorphous ferric oxyhydroxide and least onto well crystallized specular hematite. The presence of Ca or Mg at the 10?3 M level did not significantly affect uranyl adsorption. However, uranyl carbonate and hydroxy-carbonate complexing severely inhibited adsorption. The uranyl adsorption data measured in carbonate-free solutions was accurately modeled with the surface complexation-site binding model of Davis et al. (1978), assuming adsorption was chiefly of the UO2OH+ and (UO2)3(OH)+5, aqueous complexes. In modeling it was assumed that these complexes formed a monodentate UO2OH+ surface complex, and a monodentate, bidentate or tridentate (UO2)3(OH)+5surface complex. Of the latter, the bidentate surface complex is the most likely, based on crystallographic arguments. Modeling was less successful predicting uranyl adsorption in the presence of significant uranyl carbonate and hydroxy-carbonate complexing. It was necessary to slightly vary the intrinsic constants for adsorption of the di- and tricarbonate complexes in order to fit the uranyl adsorption data at total carbonate concentrations of 10?2 and 10?3 M.  相似文献   

14.
《Geochimica et cosmochimica acta》1999,63(19-20):3133-3143
  相似文献   

15.
Despite the fact that the bulk compositions of most low temperature natural surface waters, groundwaters, and porewaters are heavily influenced by alkaline earths, an understanding of the development of proton surface charge in the presence of alkaline earth adsorption on the surfaces of minerals is lacking. In particular, models of speciation at the mineral-water interface in systems involving alkaline earths need to be established for a range of different minerals. In the present study, X-ray standing wave results for Sr2+ adsorption on rutile as a tetranuclear complex [Fenter, P., Cheng, L., Rihs, S., Machesky, M., Bedyzk, M.D., Sturchio, N.C., 2000. Electrical double-layer structure at the rutile-water interface as observed in situ with small-period X-ray standing waves. J. Colloid Interface Sci.225, 154-165] are used as constraints for all the alkaline earths in surface complexation simulations of proton surface charge, metal adsorption, and electrokinetic experiments referring to wide ranges of pH, ionic strength, surface coverage, and type of oxide. The tetranuclear reaction
4>SOH+M2++H2O=(>SOH)2(>SO-)2_M(OH)++3H+  相似文献   

16.
Aqueous iodine species occur mainly as iodide (I−) and iodate (IO3−), depending on redox conditions. The adsorption of IO3− on naturally occurring oxides under oxic conditions is of environmental concern. The adsorption behaviors of IO3− by hydrous ferric oxide (HFO), α-FeOOH, and γ-Al2O3 were examined in this study as functions of pH, ionic strength, and solid concentration. Adsorption data were analyzed using an extended triple-layer model (ETLM) for surface complexation modeling to infer IO3− adsorption reactions and equilibrium constants. Results of ETLM analysis suggest that adsorption of IO3− is both an outer-sphere and an inner-sphere process, as expressed by the following complexation reactions, which are consistent with the independent pressure jump kinetic results and adsorption enthalpy measurements
  相似文献   

17.
The adsorption of hydrogen sulfide (ΓH2S) and protons (ΓH+) on the surface of crystalline sulfur was investigated experimentally in H2S-bearing solutions at temperatures of 25, 50, and 70°C, NaCl concentrations of 0.1 and 0.5 mol/dm−3 and log CH+ values in the range −2.3 to −5. At all temperatures, the dominant process on the surface of the sulfur was deprotonation, and the average values of ΓH2S were very close to the highest values determined for ΓH+. This finding, combined with the lack of detectable proton adsorption in H2S-free solutions, suggests that proton adsorption/desorption on the surface of sulfur occurs through formation of ≡ S − H2S complexes in the presence of H2S.We propose that this complexation represents sulfidation of the sulfur surface, a process analogous to hydroxylation of oxide surfaces, and that the sulfidation can be described by the reaction: ≡ S + H2S = ≡SSH20 β° The deprotonation of the ≡ SH° complex occurs via the reaction: ≡ SSH20 = ≡SSH− + H+ β− Values of 2.9, 2.8, and 2.9 (± 0.23) were obtained for −log β− at 25, 50, and 70°C, respectively. These data were employed to estimate the second dissociation constant for hydrogen sulfide in aqueous solutions using the extrapolation method proposed by Schoonen and Barnes (1988) and yielded corresponding values for the constant of 17.4 ± 0.3, 15.7, and 14.5, respectively. The value for 25°C is in very good agreement with the experimentally determined values of Giggenbach (1971) at 17 ± 0.1; Meyer et al. (1983) at 17 ± 1; Licht and Manassen (1987) at 17.6 ± 0.3; and Licht et al. (1990) at 17.1 ± 0.3.  相似文献   

18.
Two chemical processes can remove Mg2+ from suspensions containing amorphous silica (am-SiO2) at low temperatures: adsorption and precipitation of a Mg-hydroxysilicate resembling sepiolite. Mg2+ removal from am-SiO2 suspensions was investigated, and the relative role of the two removal processes evaluated, as a function of: pH, ionic strength, Mg2+ concentration, and temperature.The extent of Mg2+ adsorption onto am-SiO2 decreases with increasing NaCl concentration due to displacement of Mg2+ by Na+. At NaCl concentrations of 0.05 M and above, adsorption occurs only at pH values above 8.5, where rapid dissolution of am-SiO2 gives rise to high concentrations of dissolved silica, resulting in supersaturation with respect to sepiolite. Removal of Mg2+, at concentrations of 40 to 650 μM, from am-SiO2 suspensions in 0.70 M NaCl at 25 °C occurs at pH 9.0 and above. Experiments show that under these conditions adsorption and Mg-hydroxysilicate precipitation remove Mg2+ at similar rates. For 0.05 M Mg2+, at 0.70 M ionic strength and 25 °C, measurable Mg2+ removal occurs down to ca. pH 7.5 but is primarily due to Mg-hydroxysilicate precipitation. For the same solution conditions at 5°C, Mg2+ removal occurs above pH 8.0 and is primarily due to adsorption.Assuming that increasing pressure does not greatly enhance adsorption, Mg2+ adsorption onto am-SiO2 is an insignificant process in sea water. The surface charge of pristine am-SiO2 in sea water is primarily controlled by interactions with Na+. The principal reaction between Mg2+ and am-SiO2 in marine sediments is sepiolite precipitation.The age distribution of sepiolite in siliceous pelagic sediments is influenced by temperatures of bottom waters and by geothermal gradients.  相似文献   

19.
The dissolution of well crystallized gibbsite far at from equilibrium was studied in batch and mixed flow through reactors. The dissolution experiments were carried out between pH 2 and 6 in the presence of 10 mmol L−1 citrate, at pH 2 and 3 in the presence of 10 mmol L−1 chloride, nitrate, and sulfate, and at pH 2 and 3 in the presence of 1.5 mmol L−1 silica at 20°C. The dissolution rate of gibbsite, RAl (mol m−2 s−1), increases in the order of chloride ≈ nitrate < silica < sulfate ≈ citrate. In presence of silica, sulphate, and citrate dissolution is catalysed by the formation of aluminium complexes at the gibbsite surface (pH 2 and 3). From pH 2 to 3 no effect of RAl on hydrogen activity is predicted as singly coordinated surface sites at the edges of the platy gibbsite crystals, [≡AlOH2+0.5] ≈ [≡AlOH], are almost saturated with protons. However at pH >3 dissolution is slowed by a decrease of [≡AlOH2+0.5].Gibbsite dissolution rates measured in closed and open systems were identical within the experimental and analytical uncertainty. This observation indicates that gibbsite dissolution is a surface controlled process. If dissolution of gibbsite occurs close to equilibrium RAl values may be predicted by an approximately linear function of ΔGr.  相似文献   

20.
Sorption of Ni(II) onto chlorite surfaces was studied as a function of pH (5–10), ionic strength (0.01–0.5 M) and Ni concentration (10−8–10−6 M) in an Ar atmosphere using batch sorption with radioactive 63Ni as tracer. Such studies are important since Ni(II) is one of the major activation products in spent nuclear fuel and sorption data on minerals such as chlorite are lacking. The sorption of Ni(II) onto chlorite was dependent on pH but not ionic strength, which indicates that the process primarily comprises sorption by surface complexation. The maximum sorption was at pH ∼ 8 (Kd = ∼10−3 cm3/g). Desorption studies over a period of 1–2 weeks involving replacement of the aqueous solution indicated a low degree of desorption. The acid–base properties of the chlorite mineral were determined by titration and described using a non-electrostatic surface complexation model in FITEQL. A 2-pK NEM model and three surface complexes, Chl_OHNi2+, Chl_OHNi(OH)+ and Chl_OHNi(OH)2, gave the best fit to the sorption results using FITEQL. The high Kd values and low degree of desorption observed indicate that under expected groundwater conditions, a large fraction of Ni(II) that is potentially leachable from spent nuclear fuel may be prevented from migrating by sorption onto chlorite surfaces.  相似文献   

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